The advent of recombinant DNA technology has made it possible to produce foreign proteins in mammalian cells through the introduction of foreign DNA encoding such proteins. Mammalian expression systems are available in which the foreign protein is constitutively expressed from an active promoter. This results in the continual expression of the foreign gene. Viral promoters are commonly used as regulatory elements in gene therapy vectors due to their strong activity in various cell lines in vitro. A widely used promoter in expression systems is the human cytomegalovirus immediate-early gene (CMV) promoter. The CMV promoter induces high-level constitutive expression in a variety of cell lines (Fitzsimons et al., Methods. 28:227 [2002]).
Frequently, the abundant presence of the foreign protein is toxic to the host cell. As a result of constitutive expression, the host cell population may become moribund and perish. Furthermore, the abundant presence of the toxic protein exerts selective pressures on the host cells which can result in the emergence of a cell population containing mutated versions of the foreign DNA, which express grossly modified protein or which have deleted the foreign gene. As a result, commercially useful levels of constitutive expression may never be maintained in the recombinant cell population.
Efforts to combat this shortcoming have resulted in the development of inducible mammalian expression systems that control the expression of the foreign protein. Inducible expression can be achieved by using promoters that are controlled by the presence or absence of a specific regulator. Another means of controlling foreign gene expression involves the use of a promoter that becomes more active in the presence of a specific activator protein. A foreign gene under the control of such a promoter is expressed at high levels only following the induction of synthesis of the activator. However, many of these inducible systems currently available suffer from decreased levels of expression and “leaky” control of expression (i.e., unwanted, low-level protein expression). A more ideal inducible system would have 1) low basal expression levels; 2) high induced expression; and 3) inducer-specific, modulated expression (Xu et al., Gene. 309:145 [2003]).
Accordingly, it would be desirable to provide a tightly regulated and highly inducible protein expression system that would overcome the aforementioned and other disadvantages.